|Abstract or Summary
- Uptake, in-plant transport, and local accumulation of organic
chemicals by plants are influenced by plant characteristics, properties
of the chemical and the soil, and by environmental conditions. Evaluations
of plant contamination required by regulatory agencies cannot be
made experimentally for the many thousands of xenobiotic chemicals in
existence or being developed. A predictive simulator in the form of a
mathematical model would provide a valuable tool for such evaluations.
For this reason, a mathematical model (UTAB, uptake, Translocation,
Accumulation, Biodegradation) was formulated by defining a generic
plant as a set of adjacent compartments representing the major pools
involved in transport and accumulation of water and solutes. The model
consists of one root compartment, three stem compartments, and three
leaf compartments. Each compartment is subdivided into two transport
compartments, one for xylem and one for phloem, and a storage compartment.
In addition, two compartments model the root volume outside the
Casparian strip, one for the apparent free space and one for the cell
volume. Values for the anatomical dimensions of the compartments and
for physical and chemical coefficients were chosen from the literature.
The complete system of equations, which describes uptake and accumulation,
consists of 24 differential equations which are solved in terms
of the chemical mass in each compartment as a function of time. The
solution procedure is also developed and presented.
For calibration purposes, concentrations measured in roots, stems,
and leaves were compared with model predictions, while model parameters
were changed until no further improvement in matching model predictions with experimental results was obtained. This exercise revealed important
plant behavior that was not accounted for in the original formulation
of the model and, as such, showed the value of the model for
elucidating plant response.
The model satisfactorily predicted the observed uptake and distribution
patterns for bromacil in soybean plants, at the stage of growth
and under the environmental conditions used in the experiments, involving
a range of transpiration rates. This indicates that the model is
flexible enough to provide an accurate representation of uptake and the
influence of transpiration rate on the uptake and translocation of this
chemical. Parameter values used in the model were selected from
literature and experimental observation. They functioned well in these
simulations and they are appropriately applied in the model. The
chemical parameters for storage, mobilization, and diffusion when used
in the model also yielded satisfactory results, suggesting that they
are also appropriately applied. Finally, the calibration, although of
limited scope, showed that the model equations yielded an accurate
picture of the actual uptake patterns for bromacil in soybeans used in
these experiments. The theoretical exercise of compiling the model is
shown to be a constructive step in learning how to predict the fate of
xenobiotic contamination in plants. The model shows excellent promise
for future use. However, additional testing and validation are needed.